docs: def support_difference_count(m, m_hat):
      """Count the number of different elements in the support in one triangle,
      not including the diagonal.
      """
      m_nnz, m_hat_nnz, intersection_nnz = _nonzero_intersection(m, m_hat)
      return int((m_nnz + m_hat_nnz - (2 * intersection_nnz)) / 2.0)